Here the modification of the two sets of
organs appears to have occurred simultaneously, as far as we can judge from
their equal state of abortion. If the hermaphrodites were supplanted by the
individuals having separated sexes, and if these latter were equalised in
number, a strictly dioecious species would be formed.
There is much difficulty in understanding why hermaphrodite plants should ever
have been rendered dioecious. There would be no such conversion, unless pollen
was already carried regularly by insects or by the wind from one individual to
the other; for otherwise every step towards dioeciousness would lead towards
sterility. As we must assume that cross-fertilisation was assured before an
hermaphrodite could be changed into a dioecious plant, we may conclude that the
conversion has not been effected for the sake of gaining the great benefits
which follow from cross-fertilisation. We can, however, see that if a species
were subjected to unfavourable conditions from severe competition with other
plants, or from any other cause, the production of the male and female elements
and the maturation of the ovules by the same individual, might prove too great a
strain on its powers, and the separation of the sexes would then be highly
beneficial. This, however, would be effected only under the contingency of a
reduced number of seeds, produced by the females alone, being sufficient to keep
up the stock.
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